docs: VR-013 and VR-014 results; AR-002 raised to 40px

Records study results and the requirement change that follows from them.

VR-013 measures minimum face size end to end — gallery from one recording,
probes from another — rather than by degrading an already-aligned crop. Holding
90% of the plateau needs ~50 px that way against VR-005's ~22 px, the gap being
detection and landmark error rather than the embedder. AR-002 therefore takes
40 px, not 32: VR-005 isolates the embedder and is an upper bound, and 32 admits
faces in the falling region. FPI stayed 0.0% at every scale, and the ceiling is
cross-view rather than resolution.

VR-014 exercises audio-signature offset recovery on real film audio instead of
the synthetic golden tone. Forty random in-cap offsets, every one recovered to
the nearest frame, worst error 46 ms against a 500 ms budget — and 46 ms is the
quantisation floor rather than a result, since offsets land on whole 92.88 ms
frames. The runtime/2 anchor is confirmed through head-trimmed files.

The soft spot VR-014 found is tier labelling, not accuracy: the score drops with
sub-frame misalignment, so 27 of 40 correct alignments were demoted to `loose`.
One frame of slack in the score restores all forty to `audio` with false matches
unmoved.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

TRACES: AR-002, VR-005, VR-013, VR-014 | SR-002, SR-003

docs/SPEC.md

@@ -40,12 +40,28 @@ Detect faces in sampled video frames.
   presence (SR-002) a lower rate still answers the question, but it lengthens the
   interval between samples and so weakens IoU-based association; sweep the two
   together (VR-002).
-- **Minimum face size is 66×66 px**, expressed in **original video resolution**,
+- **Minimum face size is 40×40 px**, expressed in **original video resolution**,
   not decoded-frame pixels. Stating it in original space decouples it from
   `dense_scale`: otherwise a 0.5 downscale silently doubles the effective
   threshold, and dense mode is exactly what scene detection uses.
-  66 is a working estimate of where ArcFace embeddings stop being reliable, not a
-  measured value — it should be replaced by the result of VR-005.
+
+  40 is **measured, not estimated** — it replaces an earlier 66 px guess. Two
+  studies bracket it, and the difference between them is the whole reason the
+  number is 40 rather than 32:
+
+    - **VR-005** degrades an already-aligned 112×112 crop and matches it against
+      a native-resolution gallery. Alignment is held perfect, so it isolates the
+      *embedder*: the knee sits at 24–32 px, and 32 px still returns 98.1% TPI.
+    - **VR-013** downscales the **whole frame before the detector**, so detection
+      and landmark regression degrade along with it. End to end, holding 90% of
+      the plateau needs roughly **50 px**, against VR-005's ~22 px.
+
+  The gap is detection and landmark error, which VR-005 excludes by construction
+  — so VR-005 is an **upper bound on quality**, not a threshold, and reading a
+  floor off it would admit faces in the falling region. **AR-002 therefore takes
+  VR-013's number.** 40 sits below the 50 px plateau deliberately: FPI is 0.0% at
+  every scale in both studies, so resolution loss costs recall and never
+  precision, and an over-tight floor discards presence that SR-002 requires.
 - Emits bounding box, detector confidence, and 5-point landmarks.
 - Bounding boxes must be reported in **original video pixel space**. When
   `dense_scale < 1` downscales the decoded frame, coordinates are rescaled by
@@ -59,7 +75,9 @@ Detect faces in sampled video frames.
 **Current:** SCRFD-500MF via `face_detector_node.hpp`, thresholds in `config.hpp`
 (`detector_conf` 0.5, `detector_nms` 0.4), `min_face_px` 40, `max_faces` 10.
 
-**Gap:** `min_face_px` → 66 and re-expressed in original resolution; `max_faces`
+**Gap:** `min_face_px` re-expressed in original resolution — the value 40 is
+already correct after VR-013, so what remains is the space it is measured in, not
+the number; `max_faces`
 removed, gated on backpressure (AR-004).
 
 ## AR-004 — Backpressure
@@ -227,8 +245,8 @@ evidence. They are kept separate and **not collapsed into one scalar**: they fai
 for different reasons, have different remedies, and — as below — do not even earn
 the same response.
 
-- **Size** — already AR-002, floor at 32×32 px in original resolution, measured
-  by VR-005 (knee at 24–32 px). It is the precedent for the other two: the
+- **Size** — already AR-002, floor at 40×40 px in original resolution, measured
+  end to end by VR-013. It is the precedent for the other two: the
   threshold was *located*, not chosen.
 - **Sharpness** — motion blur and soft focus destroy the high-frequency detail
   the embedder keys on, and unlike size they leave the bounding box looking
@@ -260,6 +278,15 @@ the same response.
   canonical px. Smooth and monotone with a usable range; the mapping onto real
   faces is VR-012's to establish, and no threshold is set from these numbers.
 
+  **The synthetic ladder is noise-free and therefore optimistic about the low
+  end.** Measured on 400 real TMDB/Jellyfin headshots — the most frontal, most
+  cooperative population the pipeline ever sees — the residual runs p5 1.11,
+  median 2.74, p90 4.82, max 6.35 canonical px. So landmark noise alone occupies
+  roughly the first 3 px, and the synthetic sweep's "26° yaw ≈ 1.2 px" sits
+  *below* the noise floor on real data. VR-012 must set any threshold against
+  this measured distribution, and a discount curve has to treat the first few
+  pixels as uninformative rather than as mild pose.
+
   Neither a dedicated landmark model (`models/2d106det.onnx` is present but
   referenced nowhere — and it emits points, not pose) nor a direct pose CNN is
   adopted unless VR-012 shows the residual insufficient. If one is needed the
@@ -278,15 +305,6 @@ the same response.
 Only size drops the face outright, and only because VR-005 measured a knee below
 which the embedding carries no signal to discount. Blur and pose are different:
 
-  **The synthetic ladder is noise-free and therefore optimistic about the low
-  end.** Measured on 400 real TMDB/Jellyfin headshots — the most frontal, most
-  cooperative population the pipeline ever sees — the residual runs p5 1.11,
-  median 2.74, p90 4.82, max 6.35 canonical px. So landmark noise alone occupies
-  roughly the first 3 px, and the synthetic sweep's "26° yaw ≈ 1.2 px" sits
-  *below* the noise floor on real data. VR-012 must set any threshold against
-  this measured distribution, and a discount curve has to treat the first few
-  pixels as uninformative rather than as mild pose.
-
 - A blurred or turned face is still evidence of **presence**, which is what
   SR-002 actually asks about.
 - The tracker admits a link on position *or* identity precisely so that a face
@@ -1277,7 +1295,47 @@ plugin. Consequences to carry through:
 - Files never processed by this pipeline still get a signature from the plugin;
   the two paths coexist deliberately.
 
-**Gap:** entire requirement — no audio path exists in the pipeline today.
+**Current:** `src/audio_signature.*` implements the construction, and
+`tests/fixtures/audio/` holds the golden vector shared verbatim with the plugin
+repo, which now matches it byte for byte from C# (jRay `JR-042`/`JR-043`).
+`sae_audio` (nanobind, as `sae_embed` and `sae_kpn` are) exposes the same C++ to
+Python so a study drives the shipped code rather than a numpy port.
+
+**VR-014 measures what the golden vector cannot** — that the signature actually
+aligns a differently trimmed release, on real film audio rather than a synthetic
+tone. It does, with an order of magnitude to spare.
+
+**The accuracy question is settled and is not close.** What the offset is *for*
+is shifting scene windows, which are seconds long, so half a second of error is
+invisible; the budget is 500 ms. Over 40 random offsets inside the ±600-frame cap
+the recovered offset was the nearest frame every time — **worst error 46 ms**.
+That figure is the quantisation floor rather than a measurement of quality: the
+offset is expressed in whole 92.88 ms frames, so no correct answer can ever be
+worse than half a frame. The `runtime/2` anchor behaves as specified through real
+head-trimmed files (cutting `delta` from the head moves the window by
+`delta/2`), and both an out-of-cap offset and unrelated content are declined
+outright (0.10 and 0.07).
+
+**Where it is soft is tier labelling, not alignment.** The *score* at the correct
+offset falls with sub-frame misalignment — 0.94–0.99 when the true offset lands
+within 0.1 of a frame boundary, 0.69–0.73 at half a frame — because the two
+windows' frame grids no longer coincide. The offset stays right, but only 13 of
+40 cleared the server's 0.85 `audio` threshold and the other 27 were demoted to
+`loose`, a tier that means "possibly the same cut, degraded audio". The threshold
+was calibrated on a re-encode at *zero* offset, where the score is 1.00.
+
+The remedy is measured, not proposed (UT-108): counting a frame as agreeing if
+its peak bin matches **within ±1 frame** returns all 40 to `audio` (worst 0.906)
+while unrelated content and out-of-cap offsets stay at 0.12 and 0.16 — the gap
+that makes the threshold mean anything is untouched. It costs 81 ms of offset
+accuracy, of a 500 ms budget, because the flattened peak lets the argmax pick an
+adjacent frame. ±2 frames buys nothing further. Adopting it is a
+[server spec](../../JRay-public-server/SPEC.md) §3 change — the score is
+normative and shared by three repos — so this repo measures it and leaves the
+decision there.
+
+**Gap:** the signature is computed but **not yet emitted** into the truth file —
+that is the `IR-002` field and the coordinated `schema_version` bump.
 
 ## IR-006 — Jellyfin round-trip
 
@@ -1524,6 +1582,14 @@ Verify `landmarks`/`bbox`/`is_cut` suffice, and bump `schema_version` if not.
 Quantify where ArcFace degrades, replacing the 66×66 estimate in A1 with a
 measurement.
 
+> **Result, and its limit.** Knee at 24–32 px; 32 px returns 98.1% TPI at 0.0
+> FPI. But the probe is an already-aligned 112×112 crop, so alignment is held
+> perfect and this measures the **embedder alone** — an upper bound, not a
+> threshold. **VR-013** re-asks the question end to end, downscaling the whole
+> frame before the detector, and lands near 50 px. AR-002's floor of 40 px comes
+> from VR-013; this study is what shows how much of the gap is detection and
+> landmark error rather than embedding.
+
 **Method.**
 
 1. Select ~100 gallery actors having more than one mugshot.